Handle websocket errors in KubeExec

[orenbm]

The KubeExec class doesn't verify that there were no errors in the stream and returns the messages to
the caller. Then, the caller (in this case 'InjectClientCert') parses the messages and verifies that there were no errors.

It is better to handle the errors in the KubeExec class and raise an error if one occurred.

DoD:

• The KubeExec class raises an error in case one occurred
• The KubeExec class is aligned to the CommandClass pattern
  • Rename class to the action it performs
  • Use dependencies so we can easily mock them
  • call function tells the class's story